A switch consisting of a drive element, a spring contact and first and second wires made from shape memory material as well as first and second contact elements is known from U.S. Pat. No. 5,990,770. The drive element has a substantially T-shaped configuration and is pivoted at the foot of the T. Wires made from shape memory material are arranged at both ends of the cross of the T, the length of which changes depending on the temperature, it being possible to change the temperature by means of a current flowing through the wires. As a result heating due to the flow of current, the wires are transferred from a first phase into a second phase. The first contact element is connected to the spring contact, while the second contact element is rigid. The spring is a bi-stable snap-action spring which, actuated by the drive element, is transferred from a first stable final state into a second stable final state. The spring itself is divided into three regions by means of a U-shaped slot, the outer regions being connected to the middle freely cut tongue by means of a U-shaped spring. The drive element has an effect on the middle tongue only and, as a result of the movement of the middle tongue, the whole spring is moved backwards and forwards between two stable final states because of the effect of the U-shaped spring.
The disadvantage of this configuration is that it requires a relatively large space, the construction of the spring is extremely complicated and an additional drive element is required.
An object of the invention is to define a bi-stable electric switch, as well as a relay with such a switch, the switch and, in particular, the bi-stable snap-action spring, being very simply constructed. SUMMARY
The bi-stable electric switch of the invention uses a bi-stable snap-action spring. This is produced in that a part of the spring, which is thin or narrow in comparison to its characteristic linear extension, in other words a plate, is appropriately subjected to a sufficiently high pressure in the direction of the linear extension of the plate. The plate can then bulge or buckle to relieve the pressure.
The spring carries at least one contact element on at least one region. Lateral movements of the region of the spring which carries contact elements are associated with the buckling movement of the plate. These lateral movements are used to open or close circuits.
The snap-action spring is stable in both final states, that is, small deflections lead to springing back into the same final state. Because of to this it is also possible that the spring can apply static contact forces in both final states.
Actuating the spring to switch from one into the other final state is realized by one or more elements made from shape memory material. These drive elements each have two phases in which they exhibit different mechanical properties. When the drive elements make the transition from one into the other phase, which is achieved by a rise in temperature, due to the flow of electric current through the drive elements, they work mechanically to switch over the nonlinear spring.
It is of particular advantage that the bi-stable electric switch is very light and can be manufactured economically.
It is furthermore of particular advantage to use a spring working nonlinearly that provides contact forces in both switch states.
It is furthermore of particular advantage that the spring is configured as one piece and can be manufactured particularly easily. This is achieved in that a flat-form spring is used as a nonlinear spring, the longitudinal stress of which is established by plastic deformation of one or more regions thereof.
A particularly advantageous configuration of the flat-form spring has elongated slots by means of which it is subdivided into a plurality of plates. The plates are connected to one another at their ends. It is particularly advantageous to provide two elongated slots. By means of plastic deformation, for example bending, it is possible to shorten one or more plates of the spring. As a result a pressure is exerted on the other plates that are not shortened. These will then bulge or buckle and thereby relieve the pressure. A plastic deformation can also be configured in the form of a stamping and therefore an elongation of one or more plates of the flat-form spring. The elongated plates are then subjected to a pressure which they also relieve by bulging or buckling.
It is furthermore of particular advantage to use a trapezoidal spring as a nonlinear spring, the plates of which widen from the narrow side to the wide side of the trapezoidal spring in a constant ratio. In this case the wide side of the spring can be rigidly fixed. Using this spring form ensures a highly uniform distribution of the load. It is particularly advantageous if the width of the plates has a ratio of 1:2:1.
It is furthermore of particular advantage that the spring can be adjusted with the aid of the bends formed in individual plates of the nonlinear spring. The deflection of the spring is determined by the depth of the bend. The force that is required to switch over from one stable final state into the other stable final state is co-determined by the elastic hysteresis of the bend. As a result of the fact that the trapezoidal spring broadens towards the bottom, the opportunity arises to select the force that is required for the transition from one final state into the other final state independently, of the selected deflection, by changing the position of the bend formed, because a crimp made in the narrow region of the plates leads to a gentler switching action than a crimp made in the wide region of the plates.
It is furthermore of particular advantage that the contact elements can either be electrically connected to the spring or can be connected to the spring by means of an insulating intermediate element. Providing insulating material has the advantage that the switch arc starting from the opening contact no longer has the opportunity to spark over to the opposing fixed contact.
It is furthermore of particular advantage that the spring can be connected to the drive elements made from shape memory material via a lever. In this case at least one drive element is required for each switch state. Wires can be used as a drive element, the wires having different lengths in the two phases. The drive elements are heated by the flow of electric current and thereby transferred into the other phase. Because the wires are shortened, they exert a dynamic effect on the snap-action spring and transfer it from one stable final state into the other stable final state.
Although the drive elements are heated slowly, the snap-action mechanism ensures that the electrical contacts on the one side are opened quickly, move over to the other side by snap action and the contact force is set up suddenly.
It is furthermore of particular advantage to provide auxiliary contacts which ensure that the flow of current through the wires made from shape memory material is interrupted as soon as the switch-over movement occurs. This enables the wires to be charged with a current which, when flowing continuously through the wires, would lead to destruction of the wires but which, on account of the short duration of the flow of current, does not lead to destruction of the wires. Such high strengths of current in the control circuit allow a quick switch-over, as is typical for relays.
It is of particular advantage to use a switch according to the invention as a relay.
It is furthermore of particular advantage and constitutes a further invention to use the arrangement as a polarity reversing switch. This inventive use of the inventive bi-stable switch is possible as a result of the particular configuration of the snap-action spring and the contact arrangement. In this case it is particularly advantageous to form the snap-action spring from two single springs, which are connected to each other with dimensional stability by means of nonconductive elements, as double electrically separated snap-action springs. The two single springs each constitute the center contact of a changeover switch contact arrangement which move between two fixed contacts.